The problem of icing on aerofoils is well known in the aeronautics industry. The shape of the aerofoils may be altered on account of the formation of ice that occurs because during flight, the aerofoil encounters droplets of supercooled water contained in the atmosphere.
This problem is often dealt with by equipping the aerofoil with a Joule-effect heating structure.
Usually, the heating structure consists of metal resistors. These metal resistors pose problems of mechanical integrity, particularly for aerofoils that are made of composite material, of tolerance to damage (multiple redundancy is needed to ensure that the breakage of one metal resistor does not prevent the entire device from functioning), of uniform heating per unit area, and of corrosion.
In order to limit the occurrence of these problems, it has been proposed that use be made of a composite deicer in which the resistive elements are composed of carbon fibers (see French Patent 2 578 377).
A distinction is made between "deicers", in which the resistive elements that dissipate heat are powered intermittently to remove the ice that regularly forms, and "anti-icing devices", in which the resistive elements are powered continuously to prevent ice from forming.
It will be understood that as regards the effectiveness of the fight against ice, the anti-icing device, which has a preventive action is preferable to the deicer, which has a curative action. However, the anti-icer has the drawback of consuming a great deal of electrical power when a large aerodynamic surface needs to be kept at a high enough temperature to prevent the formation of ice. Thus, the principle of anti-icing devices is often reserved for relatively small areas. For larger areas, the deicer provides a compromise between electrical power consumption and effectiveness of the heating. In general, it is enough for the deicer to be able to melt some of the ice, the rest being torn off by centrifugal force or the aerodynamic flow.
In the case of medium or heavy helicopters, a design often adopted consists in fitting the blades of the counter-torque rotor with anti-icing devices and the blades of the main rotor with deicers. These deicers all use the principle of heating resistors arranged along the span of the blade with return paths for the electrical current so as to return the electrical current from the tip to the root of the blade. Whatever their means of electrical supply (3-phase or rectified DC current) and however the heating resistors are produced (metal, carbon fibers, etc), today's deicers use either braids or strips or cables of very low electrical resistance (a few mW) as their return path. This low resistance is obtained by selecting materials with very low resistivity (tinned copper, brass, r=8.times.10.sup.-6 W.cm). This is because the requirement is for these current return paths to avoid dissipating heat energy so that all of the power is available for the heating resistors. The major drawback of these return paths results from the poor fatigue properties of the materials of which they are made, which restrict the life and lower the reliability of the complete deicer.